MEANS TO PREVENT SKIDDING EXPLAINED
B.J. Lemon, United States Tire Company.
December 24, 1922
Rear Wheels Slip Easily on Quick Application of Brake Pressure.
Why an automobile skids is not of interest to a motorist, but what to do to prevent skidding and what to do when the car is skidding are equally important topics.
The main cause of skidding of rear wheels of motor cars is due to the action of the brakes operating through the rear wheels. Skidding, of course, normally occurs only when the roads are slippery.
An automobile wheel rotates and progresses over the highway in but one direction or plane. This direction for the rear wheels is determined by the front wheels under control of the driver. While the rear wheels grip the ground and rotate they control the direction of the car. However, as soon as the rear wheels are locked by the brakes they begin to slide, and as they slide sidewise practically as easily as forward they lose power to direct the car.
WHEELS SLIDE EASILY.
It takes comparatively little brake friction to lock and slide rear wheels on slippery roads because there is little friction between the tire and the road surface. When once the driving wheels begin to slip the car is kept in motion by the force of inertia acting at the car's center of gravity or mass. This forward motion is opposed by the resistance of all four wheels on the ground, a resistance that is greater in the rotating front wheels than in the sliding rear wheels. Unless the combined ground resistance of all wheels, tending to stop the car, acts on the same point as the force of inertia tending to carry the car forward, there results what is called a "force couple" which causes the car to skid or spin around.
After the wheels once start to slip a very small "force couple" will produce skidding, and the skid distance is dependent upon the magnitude of the "force couple" which in turn depends on car speed and weight. Turning the front wheels in the direction of the rear wheel skid helps to reduce skidding because it reduces the magnitude of the "force couple." Also periodically releasing and reapplying the brake so the rear wheels can get a grip offsets forward slip and consequent tire skid.
Two means are employed to prevent skidding in motor vehicles. These are the use of non-skid or rough tread tires and of anti-skid chains. Non-skid tires are designed to give forward traction and to prevent side slip. As long as there is traction the rear wheels will turn, and as long as the rear wheels turn there will be no serious side slip, unless the front wheels are blocked.
Non-skid tires furnish reliability and safety in driving—reliability, in permitting the vehicle to operate successfully over almost any kind of road and during all seasons of the year, and safety by permitting the vehicle to hold the road at all speeds, and allowing the brakes to be most effective.
Non-skid treads on tires were used first on rear wheels and now are considered absolutely essential for such uses. Later non-skid tires were placed on front wheels because the treads prevented front wheel skidding, as well as permitted the rotation or exchange of the front and rear tires.
A well designed non-skid tread should give on rear wheels sufficient forward traction to permit the car to operate without chains under most road conditions and at the same time insure against side slip. It must permit quick and smooth response to clutch, accelerator and brakes. It must not impair greatly ease of steering when used on front wheels. It must not set up vibration in the car nor produce disagreeable humming noises. It should present an attractive appearance. The Royal Cord Tread is regarded as the best example of an all around efficient and acceptable non-skid tread design.
Non-skid chains accomplish the same purpose as the non-skid tire tread, that is, give additional traction and reduce side slip. Non-skid chains should supplement non-skid tires only when road conditions absolutely require them.
Because they make available in road friction the total weight of the car instead of only half the car weight, four wheel brakes are coming into use, especially in Continental Europe. Not only can cars be stopped more quickly but they can be driven safely on wet days and in ice and snow without chains. In stopping a car equipped with four wheel brakes all four wheels may slide and the car as a whole slip forward. However, there appears to be little or no side skidding. This is especially true if the brakes are connected diagonally in pairs, that is, left hand front wheel and right hand rear wheel. With this arrangement opposite wheels are able to roll freely, retaining their directing qualities.
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